As a result of the shrinking sizes of integrated circuits, photolithographic masks have to project smaller and smaller structures onto a photosensitive layer i.e. a photoresist dispensed on a wafer. In order to fulfill the decrease of the critical dimension (CD) of the structure elements forming the integrated circuits (ICs), the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet across the mean ultraviolet into the far ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of photolithographic masks with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In the future, photolithographic masks will use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (approximately 13.5 nm).
The shrinkage of the CD for smaller actinic wavelengths requires a respective reduction of the critical dimension variation, i.e., the critical dimension uniformity (CDU) across the mask. The distribution of the CD across the mask area is directly linked with a variation of critical IC parameters fabricated within the illumination field of the mask at various positions on the wafer. Thus, an increase in the variation of the critical dimension may immediately lead to a yield reduction of the fabricated ICs.
Therefore, in addition to the CD, the variation of the CD or the CD uniformity (CDU) is a key characteristic of a photolithographic mask. It is therefore very important to know the distribution of CD across a photolithographic mask. In case the CDU of a produced mask does not fulfill a predetermined specification, the CD variation can be reduced in a CD correction process as explained in the applications U.S. provisional patent application 61/351,056 and U.S. provisional patent application 61/363,352 of the applicant which are incorporated herein by reference in their entirety.
Since some time, it has been detected that the CDU may deteriorate during the operation of the photolithographic mask in an illumination system. PCT application WO 2009/007977 A2 of the applicant and the U.S. Pat. No. 6,614,520 B1 describe that photomasks may degrade during the operation even if they have been free of defects at the beginning of their operation lifetime and disclose methods for monitoring this process. Furthermore, for example, the article “Detection of Progressive Transmission Loss Due to Haze with Galileo™ Mask DUV Transmittance Mapping Based on Non Imaging Optics” of S. Labovitz et al., BACUS Symposium on Photomask Technology, Vol. 7122 (2), 7-10 Oct. 2008, Monterey, Calif., USA, 2008, describes various causes for a broadening of the CD distribution during operation of the mask in the factory. Consequently, it is necessary to regularly control the CDU behaviour across the mask in order to detect the occurrence of a CD variation relevant defect as soon as possible, and thus avoiding yield problems for the fabricated ICs.
As an example, for the 45 nm technology node, the CD on the wafer amounts to 45 nm and the allowed CDU across the wafer are 4.7 nm for memory elements and 1.9 nm for logic (3σ). This requires a resolution in the determination of the CDU below 1 nm. Available tools which can spatially resolve structures of pattern elements in the sub-nanometer range include, for example, a scanning electron microscope (SEM) and an atomic force microscope (AFM).
However, the application of these tools is restricted to the investigation of a small number of specific mask positions as the alignment and the scan of a SEM, and/or of an AFM across the defective portion is a time-consuming process. Moreover, the application of a SEM or of an AFM for CD measurements requires the removal of the pellicle from the mask which introduces an uncertainty in the CDU determination as the influence of the re-mounting of the pellicle is ignored. Additionally, high energy electrons of the SEM may deteriorate the performance of the mask. Thus, these tools may be used for test and calibration purposes, but they are not suited for mapping the CD variation across a complete photolithographic mask.
The paper “In-field CD uniformity control by altering transmission distribution of the photomask, using ultra-fast pulsed laser technology” by Y. Morikawa et al., Photomask and Next-Generation Lithography Mask Technology XIII, Ed.: M. Hoga, Proc. Vol. 6283, May 20, 2006, describes that a variation of the optical transmission within a mask results in a proportional variation of the CD. A variation of the optical transmission or of the exposure dose change of 1% results in a CD variation of 1 to 2 nm depending on the exposure and the process conditions. Thus, optical metrology tools have to have an optical transmission resolution limit of less than 0.5% to provide the resolution required to measure the CD distribution across a photolithographic mask.
Optical measurements of the CD distribution or CD variation by analyzing the variation of the optical transmission using imaging optics are limited in their resolution. Using an imaging camera detection system (e.g., charge-coupled device (CCD camera), the CCD noise on the one hand and the limited dynamic range of a CCD camera on the other hand restrict the detection of a local optical transmission variation to an optical intensity change of about 1%. This interrelationship is for example reported in the article “Very High Sensitivity Mask DUV Transmittance Mapping and Measurement Based on Non Imaging Optics” by G. Ben-Zvi et al., Proc. 24th European Mask and Lithography Conference, Jan. 21-24, 2008.
Thus, optical metrology tools having an imaging optic and using the aerial image in the wafer plane requires some averaging of each image in order to reach the necessary resolution. As a consequence, the measurement of the optical transmission of the overall mask area with an imaging tool can be a time-consuming process. More importantly, critical dimension scanning electron microscope (CD SEM) measurements are necessary in order to determine the proportionality constant between transmission variation and CD variation at selected positions across the photomask.
Since some years ago, an optical inspection tool called Galileo™ is available that allows scanning the overall area of a photolithographic mask within a reasonable time period (cf. e.g. “Very High Sensitivity Mask DUV Transmittance Mapping and Measurements Based on Non Imaging Optics” of G. Ben-Zvi et al., Proc. 24th European Mask and Lithography Conference, Jan. 21-24, 2008). This tool uses a non-imaging optics which compromise image fidelity by allowing all scrambled angles of illumination to pass through the mask and to be detected by the detection system. With a proper non imaging optical design a large gain in optical transmission from the source to the detector can be realized, which leads to significant improvements of the signal-to-noise ratio (SNR) and thus leads to a significant reduction of the measurement time. The non-imaging metrology tool uses a broad band DUV optical light source with a beam width or spot size in the range of 0.1 mm to 5.5 mm and a fast photo detector, such as for example a photodiode or a photomultiplier tube. It can resolve optical transmission changes of less than 0.05%. By needing less than one second per measurement position, this tool allows scanning of the active area of a mask in less than one hour. The pellicle does not need to be removed for the measurement.
PCT application WO 2009/083606 A1 of the applicant describes the determination of the CDU on a mask by measuring the DUV transmission across the photolithographic mask with the non-imaging metrology tool described above. PCT application WO 2009/083606 A1 proposes to determine the proportionality constant between transmission variation and CD variation either from a linear regression of the CD variation obtained from an aerial image, from CD SEM calibration measurements, from IC manufacturing data, or from data stored in a central data base. This data is adapted to the specific photolithographic mask by calculating an individual proportionality constant for each measurement position.
The method disclosed in WO 2009/083606 A1 has some drawbacks. It requires a specific non-imaging tool in addition to the already available imaging metrology tools. Further, some effort is necessary to establish a CDU map for the photomask from the measurement of the transmission distribution caused by the determination of the proportionality constant between optical transmission variation and CD variation. The identification of the constant may comprise CD SEM and/or AFM measurements. Moreover, the CD variation determined with a non-imaging tool refers to the CD variation of the photomask. However, the more important quantity is the CD variation on the wafer which is affected the CD variation of the mask, the signature of the scanner and/or stepper used to scan the photomask across the wafer and the mask error enhancement factor (MEEF).
It is therefore one object of the present invention to provide a method and an apparatus for determining a critical dimension variation of a photolithographic mask, which at least partially removes the above mentioned drawbacks of the prior art.